Powder metallurgy processes involve use of a metal powder as a raw material. The metal powder can be pure metal or several powders of various metals can be blended together to form alloys. The metal powder can be formed to near net shape by placing it into a mold (usually made of types of rubbers) of desired size and shape. The mold is first placed into a steel container similar in size and shape to the mold in order to retain the mold's shape when filled with powder. The mold is then sealed by either mechanical means or liquid rubber. The assembly is then isostatically pressed (pressure from all directions) using an oil or water medium with pressures from 15,000 to 60,000 psi. The part is compacted and is in the as pressed or "green" state. It can be handled but will crack and chip if dropped and jarred. Therefore it is heated in a furnace at a temperature below its melting point to densify and strengthen it. This process is called sintering and involves a coalescing of powder particles into a metallic structure. There are basically two types of sintering. Solid state sintering is associated mainly with pure metals and some alloys. Liquid phase sintering is tied to only alloys and it involves the melting of the metal powders that are the alloying elements. The alloy is heated above the melting point of the alloying elements but below the melting point of the main element of the alloy. This liquid phase of alloying elements surrounds the base material and diffusion and bonding occur between the liquid phase and the base material. Thus, this type of material approaches near 100% theoretical density while solid state sintering normally produces a material with a maximum of 97% of the theoretical density.
It is sometimes necessary due to size limitations of the equipment in forming near net shape liquid phase sintered parts, to bind smaller size liquid phase sintered together to make the larger part.
The present invention invention provides a method for joining such parts by a diffusion bonding technique.